Problem: If $x \boxdot y = 3x-y$ and $x \bigtriangleup y = 2x-8y$, find $5 \bigtriangleup (-2 \boxdot -6)$.
First, find $-2 \boxdot -6$ $ -2 \boxdot -6 = (3)(-2)-(-6)$ $ \hphantom{-2 \boxdot -6} = 0$ Now, find $5 \bigtriangleup 0$ $ 5 \bigtriangleup 0 = (2)(5)-(8)(0)$ $ \hphantom{5 \bigtriangleup 0} = 10$.